196 Bulletin 194 



The first layer on the outside is probably rough externally and dark- 

 colored. This is the bark, (a, fig. VIII), a protective covering to the 

 inner woody tissues. The outer portion of the bark of a forest tree is 

 dead tissue which is stretched and fissured as the tree grows in diameter, 

 while the inner portion is live tissue, serving to conduct food materials 

 up and down the trunk. In some kinds of trees the bark is quite thick, 

 in others very thin. This factor, together with those of color and ex- 

 ternal appearance, makes the bark, when present, of considerable value 

 in the determination of woods. At the center of the log is a small 

 cylinder of loose tissue less than one-fourth of an inch in diameter, 

 the pith (g, fig. VIII). Lying between the bark and the pith is the 

 wood. In some kinds of trees, like the balsam fir and the hemlock, the 

 wood is all of one color. In others, it consists of two parts distinct 

 from each other in color. The outer portion is called sapwood (b, 

 fig. VIII). It is living tissue and serves chiefly in the living tree to 

 conduct the sap, or water which is taken up by the roots, to the leaves. 

 The width of the sapwood varies with different species and with dif- 

 ferent individuals of the same species. In very young trees all the 

 wood is sapwood. As the sapwood grows older, the tissue composing 

 it becomes choked so as to prevent the further passage of sap. This 

 tissue then dies and serves only as a support to the rest of the tree, 

 holding up its great weight and making it rigid enough to resist strong 

 winds. It usually becomes darker colored than the sapwood, and is 

 called heartwood (c, fig. VIII). The sapwood and heartwood differ 

 not only in color, but also in durability. In some species only the 

 heartwood is suitable for sawing into lumber, being, as a rule, harder, 

 heavier and drier than sapwood. In other cases where pliability or 

 elasticity is required, as, for example, in hickory and ash which are 

 used in making baskets or for handles of rakes, hoes and golf clubs, the 

 sapwood is more valuable than the heartwood. 



Separating the wood from the bark is a very thin layer, the cam- 

 bium. From this layer is added new wood on the outside of that 

 already formed and new bark on the inside of the old bark. It is by 

 this process that the trunk grows in diameter. Each year the cambium 

 adds a new layer of wood to the circumference of the tree. These 

 layers appear on the end of the log as concentric rings. They are called 

 annual, yearly or growth rings (h, fig. VIII). In maple, birch, white 

 pine and many other woods there is no sharp distinction between the 

 wood formed in the spring and that formed in summer. In other 

 cases the wood which is formed in the spring, when growth is most 



